Civil libertarians are raising the alarm over the state’s plans to create a Big Brother database that could map drivers’ whereabouts with police cruiser-mounted scanners that capture thousands of license plates per hour — storing that information indefinitely where local cops, staties, feds and prosecutors could access it as they choose.

“What kind of a society are we creating here?” asked civil rights lawyer Harvey Silverglate, who along with the ACLU fears police abuse. “There comes a point where the surveillance is so pervasive and total that it’s a misnomer to call a society free any longer.”

The computerized scanners, known as Automatic License Plate Recognition devices, instantly check for police alerts, warrants, traffic violations and parking tickets, which cops say could be an invaluable tool in thwarting crime. The Executive Office of Public Safety has approved 27 grants totaling $500,000 to buy scanners for state police and 26 local departments. The purchases are on hold while state lawyers develop a policy for the use of a common state database all the scanners would feed.Some ALPR scanners already are deployed on Massachusetts roads. State police have two. Several cities use them for parking enforcement. Chelsea has four scanner-mounted cruisers.

“It’s great for canvassing an area, say after a homicide if you are looking for a particular plate,” said Chelsea police Capt. Keith Houghton. “You can plug it in, and drive up and down side streets. It sounds an alarm if you get a hit.”

He said Chelsea’s information is overwritten after 30 days and is not shared with the state.EOPS spokesman Terrell Harris said the state wants the scanner information fed into the Public Safety Data Center, where local, state and federal authorities could access it.

“We’re currently working to develop a policy that balances the effective use of this powerful law enforcement tool with the privacy concerns we’re keenly aware of,” Harris said.

The ACLU’s Kade Crockford said the technology, which just allows a faster version of what police do now in running plates, is less of a concern than the state’s plans to store information on average, law-abiding citizens.

“People who aren’t wanted for a crime, all of their information is stored in a database that is shared with another government agency,” Crawford said. “The potential for abuse is very big. We don’t think people who haven’t committed a crime should be tracked by law enforcement.”

The two state police cruisers equipped with scanners patrol the metro Boston area, state police spokesman David Procopio said. He defended police use of the new technology.

“What about the rights of someone who is already a victim to have their assailant brought to justice?” Procopio asked. “There’s a freedom to being able to live your life not worried about being the victim of crime that’s also a freedom worth protecting."

Silverglate countered, “If you have cameras everywhere, of course you’re going to reduce the crime rate, but you’re not going to have a society worth preserving. To the American people, freedom means something. There is a line to draw in the sand, beyond which you don’t want the government poking its nose. This crosses the line.”